The present invention generally relates to color toners and image forming methods, and, more particularly, to a color toner used for image formation using a technique such as electrophotography and a method of forming an image on a recording medium with the color toner. Furthermore, the present invention relates to a color toner that is fixed onto a recording medium by the light energy originating from flashlight, and an image forming method using the color toner.
Electrophotography is a technique widely used for image forming apparatuses such as copying machines, electrophotographic facsimile machines, and electrophotographic printers. As disclosed in U.S. Pat. No. 2,297,691, an electrophotographic technique using a photoconductive insulating body has been widely used. According to this technique, an electrostatic latent image is formed by emitting light, such as laser or LED, onto a photoconductive insulating body charged by corona discharge or a charge supply roller. Resin particles called toner that is colored with pigments or dyes (coloring agents) are electrostatically fixed to the electrostatic latent image to carry out a development to obtain a visible toner image. This toner image is then transferred onto a recording medium such as paper or film. At this point, the toner image is only placed on the recording medium, and, therefore, needs to be fixed to the recording medium. Accordingly, the toner is melted on the recording medium by heat, pressure, or light, and are then solidified. Thus, a toner image fixed onto the recording medium can be obtained.
As described above, the toner fixing is carried out by melting a toner mainly made up of thermoplastic resin (hereinafter referred to as “binder resin”) and then fixing the melted toner onto the recording medium. Well-known examples of such toner fixing methods include a heat-roll technique in which a recording medium having a toner image formed thereon is heated and pressurized directly by a heated roller, and a flash fixing technique in which the toner is fixed onto a recording medium by flashlight from a xenon flash tube, for instance.
According to the flash fixing technique, the toner is melted and fixed onto a recording medium by converting light energy originated from flashlight of a discharge tube such as a xenon flash tube into thermal energy.
This flash fixing technique has advantages over the heat-roll technique in an image forming apparatus. The advantages include: (1) the resolution of a toner image formed on a recording medium does not deteriorate by the non-contact fixing; (2) no warm-up time is necessary for the image forming apparatus, and an image forming operation can be promptly started; and (3) the fixing can be carried out on recording media of any material or thickness, such as adhesive paper, pre-printed paper, and paper having various thicknesses (where the thickness varies stepwise).
The process of fixing a toner onto a recording medium by flashlight are as follows. Flashlight emitted from a discharge tube is absorbed by a toner image (a particle image) formed on the recording medium, and is then converted into thermal energy. The toner is heated up accordingly, and is melted. As a result, the toner adheres to the recording medium. After the end of the flashlight emission, the temperature drops, and the melted toner solidifies to form a fixed toner image.
Preferable conditions for carrying out the flash fixing includes:
1) The toner is capable of efficiently converting light energy into thermal energy,
2) The material of the toner can melt with heat and turn into a melted state that is permeable to the recording medium, and
3) Little energy is lost on anything other than the melting of the toner material. (For instance, energy loss due to heat absorption by the recording medium is not desirable.)
However, a xenon flash tube that is generally used as a discharge tune for flash fixing emits light over a wide wavelength range of 400 nm to 2000 nm. Compared with light in the visible range of 400 nm to 800 nm, the luminescence intensity in the near-infrared range of 800 nm to 2000 nm in wavelength, especially in the range of 800 nm to 1400 nm, is very high. For this reason, the toner on which flash fixing is to be carried out is required to have a high light absorptivity for light in the near-infrared range of 800 nm to 2000 nm in wavelength, especially in the 800 nm to 1400 nm range.
However, binder resin that is the main component of a toner generally has a very low light absorptivity in the visible range and the near-infrared range. When the coloring agent is black, the toner exhibits a high light absorptivity in the visible range and the near-infrared range. When the coloring agent is for a color, such as yellow, cyan, magenta, red, blue, or green, the toner exhibits an acceptable light absorptivity in the visible range, but has only a low light absorptivity in the near-infrared range. This implies that a color toner has a poor light-to-heat conversion efficiency.
For the above reasons, it is difficult to fix a color toner containing binder resin and coloring agents by flashlight of such intensity as to fix a black toner. Color toner fixing therefore requires the supply of strong light energy.
There have been techniques of adding an infrared absorbent having a high light absorptivity in the light emission wavelength range of a xenon flash tube to a color toner, so that the required amount of light energy for fixing the color toner onto a recording medium by flashlight can be reduced. For instance, Japanese Laid-Open Patent Application Nos. 61-132959, 6-118694, 7-191492, and 2000-147824 each disclose a technique of adding an aminium compound, a diimonium compound, or a naphthalocyanine compound to a toner for flash fixing. Japanese Laid-Open Patent Application No. 6-348056 discloses a technique of applying resin particles containing an infrared absorbent containing anthraquinone dye, polymethine dye, or cyanine dye, to the toner surface. Japanese Laid-Open Patent Application No. 10-39535 further discloses a technique of increasing the fixability of a color toner with flashlight by adding tin oxide or indium oxide to the color toner.
In the above disclosed techniques, an infrared absorbent is added to a color toner, so as to promote the efficiency of converting light energy into thermal energy, and to increase the meltability of the binder resin as the main component of the color toner.
However, the addition of an infrared absorbent is not enough to solve the problem, i.e., not enough to melt the binder resin contained in a toner. Also, the aminium compound, the diimonium compound, and the naphthalocyanine compound used as the preferred infrared absorbent is colored, and a large amount of those compounds will have adverse influence on a fixed color image. Accordingly, it is preferable to use only a small amount of infrared absorbent.
As described so far, a large amount of light energy is still required for fixing a color toner by flashlight.
In an image forming apparatus that can form a color toner image, a plurality of color toners of different colors, instead of a single color toner, are used for twin-color image formation, multi-color image formation, or full-color image formation.
In twin-color image formation, multi-color image formation, and full-color image formation, a plurality of color toners and a black toner are simultaneously used. However, if the melt characteristics and light absorption characteristics greatly differs among the colors being simultaneously used, there is a problem that the glossiness of the image fixed by light varies with the color tones. If the characteristics differences are very large, some colors of the toner are insufficiently fixed while some other colors are sufficiently fixed. Furthermore, voids due to excessive absorption of light energy might lead to defects in a formed image.